Myelodysplastic syndromes (MDS) are heterogeneous clonal hematopoietic stem and progenitor cell (HSPC) disorders with frequent transformation to acute myeloid leukemia (AML). Molecular events leading to AML transformation are poorly defined and the only curative therapy is allogeneic stem cell transplantations, for which few patients are eligible. One of the reasons for ineffective treatments is the lack of understanding of the mechanisms of disease. Therefore, investigations into the cellular and molecular basis of genetically- defined MDS and AML subtypes are crucial to the development of novel therapies. Deletions on chromosome (chr) 5q (del5q) are the most frequent cytogenetic abnormality in MDS and are indicative of poor prognosis in AML. However, the functions of individual genes within del(5q) are largely unexplored. We recently reported that TRAF-interacting protein with forkhead-associated (FHA) domain B (TIFAB) is deleted in all reported cases of del(5q) MDS/AML and its deletion in mice results in phenotypes reminiscent of human MDS, due partly to aberrant innate immune signaling via TRAF6 activation; but, we also observed increases in p53 activity. To fully elucidate the mechanism of TIFAB-dependent regulation of p53, we performed a proteomics screen for TIFAB-binding proteins. Ubiquitin-specific peptidase 15 (USP15), a deubiquitinase that stabilizes MDM2 and enforces p53 degradation, emerged as the lead candidate. Therefore, we devised a multi-pronged approach in which we first, characterized the TIFAB-dependent p53 activity in our Tifab-deficient MDS model (Aim 1A), and second, uncovered the molecular mechanism of the TIFAB-USP15 complex in regulating p53 (Aim 1B). Our findings garnered rationale for defining the function of USP15 in established malignant hematopoiesis, which is well underway and which we propose to complete during the F99 phase of this research program (Aim 1C). Our preliminary data indicate that USP15 is highly expressed in AML and is important for leukemic progenitor function, but not normal HSPCs. We will employ a combination of in vivo and in vitro approaches using our Usp15-/- model, MLL-AF9 leukemia models, and AML cell lines to elucidate the function of USP15 in AML. In completing my dissertation research, I have acquired a strong foundation in mouse genetics, murine models, hematopoietic malignancies including MDS and AML, cancer biology, and cellular mechanisms. I have learned biochemical assays, structure-function studies, and genetic studies, while gaining familiarity with screening approaches including RNA-sequencing and proteomics. With my comprehensive training, I am positioned and highly motivated to pursue a post-doctoral research direction elucidating the determinants of USP15 activity, while utilizing drug discovery platforms to develop small- molecule inhibitors against this druggable target (Aim 2).